Light emitting diode having light emitting cell with different size and light emitting device thereof

ABSTRACT

There is provided a light emitting diode operating under AC power comprising a substrate; a buffer layer formed on the substrate; and a plurality of light emitting cells formed on the buffer layer to have different sizes and to be electrically isolated from one another, the plurality of light emitting cells being connected in series through metal wires. 
     According to the present invention, light emitting cells formed in an LED have different sizes, and thus have different turn-on voltages when light is emitted under AC power, so that times when the respective light emitting cells start emitting light are different to thereby effectively reduce a flicker phenomenon.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the National Stage of International Application No.PCT/KR2007/004449, filed Sep. 14, 2007, and claims priority from and thebenefit of Korean Patent Application No. 10-2006-0096759, filed on Sep.30, 2006, which are both hereby incorporated by reference for allpurposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light emitting diode having lightemitting cells with different sizes and a light emitting device havingthe same.

2. Discussion of the Background

A light emitting diode (LED), which is a photoelectric conversion devicehaving a structure in which an N-type semiconductor and a P-typesemiconductor are joined together, emits predetermined light throughrecombination of the electrons and holes. Such an LED is widely used fordisplay elements and backlights. Further, LEDs have less electric powerconsumption and a longer lifespan as compared with conventional lightbulbs or fluorescent lamps, so that their application areas have beenexpanded to the use thereof for general illumination while substitutingfor conventional incandescent bulbs and fluorescent lamps.

An LED is repeatedly turned on/off depending on the direction of currentunder an AC power source. Hence, when the LED is directly connected toan AC power source, the LED may not continuously emit light and may beeasily damaged due to reverse current.

To solve such a problem, an LED capable of being connected directly to ahigh-voltage AC power source is disclosed in PCT Patent Publication No.WO 2004/023568(A1), entitled “LIGHT-EMITTING DEVICE HAVINGLIGHT-EMITTING ELEMENTS” by SAKAI et al.

According to PCT Patent Publication No. WO 2004/023568(A1), LEDs aretwo-dimensionally connected in series on an insulative substrate such asa sapphire substrate to form LED arrays. Such two LED arrays areconnected to each other in reverse parallel on the sapphire substrate.As a result, there is provided a single-chip light emitting diodecapable of being directly driven by an AC power supply.

FIG. 1 is a view showing an arrangement of light emitting cells in aconventional LED.

Referring to FIG. 1, a conventional LED 10 performs light emittingoperation by AC power supplied from a power supply 20 for supplying ACpower.

The LED 10 comprises a plurality of light emitting cells 11 arranged intwo parallel rows, where first and second rows are arranged to havepolarities opposite to each other.

Hence, if AC power is applied from the power supply 20, current flowsinto the first row in positive voltage intervals such that the lightemitting cells 11 in the first row emit light, and current flows intothe second row in negative voltage intervals such that the lightemitting cells 11 in the second row emit light.

Therefore, the first and second rows alternately emit light.

The respective light emitting cells 11 of the LED 10 are formed on onesubstrate through the same process. The respective light emitting cells11 in the LED 10 are formed to be electrically separated from oneanother on the substrate and then connected electrically to one anotherby metal wires.

At this time, the respective light emitting cells 11 in the LED 10 havethe same size. Hence, the respective light emitting cells 11 have almostthe same turn-on voltage as shown in FIG. 2. When power is applied torespective light emitting cells, the turn-on voltage of each lightemitting cell is determined depending on the current density of acorresponding light emitting cell, and the respective light emittingcells 11 in the LED 10 are formed on the one substrate through the sameprocess to have the same size. For this reason, the respective lightemitting cells 11 in the LED 10 have the same current density.

If AC with a frequency of 60 Hz is applied as the respective lightemitting cells 11 in the LED 10 have the same turn-on voltage, the lightemitting cells 11 are periodically turned on by the applied AC tothereby emit light.

That is, the respective light emitting cells 11 are turned on to emitlight if the voltage applied by the AC with a frequency of 60 Hz is overthe turn-on voltage, while the respective light emitting cells 11suspend emitting light if the voltage applied by the AC with a frequencyof 60 Hz is below the turn-on voltage.

At this time, as the respective light emitting cells 11 have the sameturn-on voltage, all the light emitting cells 11 are turned on at acertain time. Then, the respective light emitting cells 11 stop emittinglight at the instance when the voltage drops below the turn-on voltageafter the light emitting cells 11 are turned on at the same turn-onvoltage, which causes a flicker phenomenon to occur.

Such a flicker phenomenon may not be easily visible with the naked eyes.However, in the field of illuminator using AC power with a plurality oflight emitting cells provided, a stable light source is required inreducing the flicker phenomenon if possible.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an LED having aplurality of light emitting cells capable of reducing a flickerphenomenon generated because the turn-on voltages of the respectivelight emitting cells are similar to one another.

According to an aspect of the present invention, there is provided alight emitting diode (LED) operating under AC power, comprising asubstrate; a buffer layer formed on the substrate; and a plurality oflight emitting cells formed on the buffer layer to have different sizesand to be electrically isolated from one another, the plurality of lightemitting cells being connected in series through metal wires.

Preferably, the plurality of light emitting cells are arranged so thatadjacent ones of the respective light emitting cells electricallyconnected have different sizes and are repeated.

Preferably, the plurality of light emitting cells are arranged in twoparallel rows, and polarities of the first and second rows are arrangedto be opposite to each other.

More preferably, the light emitting cells arranged in each row havefirst and second sizes and alternately arranged adjacent to each otherin the corresponding row, and when the light emitting cell formed at aposition in the first row has the first size, the light emitting cellformed at a position in the second row corresponding to the position inthe first row has the second size.

Preferably, each of the light emitting cells comprises an N-typesemiconductor layer, an active layer and a P-type semiconductor layer,and the N-type and P-type semiconductor layers of adjacent ones of thelight emitting cells are electrically connected in series through themetal wires.

According to another aspect of the present invention, there is provideda light emitting device having a plurality of LEDs arranged to operateunder AC power, wherein each of the LEDs comprises a substrate; a bufferlayer formed on the substrate; and a plurality of light emitting cellsformed on the buffer layer to have different sizes and to beelectrically isolated from one another, the plurality of light emittingcells being connected in series through metal wires.

Preferably, the respective LEDs comprise the same substrate in common.

According to the present invention, light emitting cells formed in anLED have different sizes, and thus have different turn-on voltages whenlight is emitted under AC power, so that times when the respective lightemitting cells start emitting light are different to thereby effectivelyreduce a flicker phenomenon.

Further, the plurality of light emitting cells in an LED are arranged sothat adjacent ones of the respective light emitting cells electricallyconnected have different sizes and an arrangement of the respectivelight emitting cells having different sizes are repeated, so that theintensity of light emitted from the LED is not largely differentdepending on positions in the LED, thereby uniformly emitting lightthroughout the LED.

Furthermore, a plurality of light emitting cells in an LED are arrangedin two parallel rows, and the first and second rows are arranged so thatthe polarities thereof are opposite to each other, whereby a lightemitting period of the LED can be more shorten when the LED emits lightunder AC power to thereby effectively reduce a flicker phenomenon.

In addition, in a state where a plurality of light emitting cells arearranged in two parallel rows and the first and second rows are arrangedso that the polarities thereof are opposite to each other, the lightemitting cells arranged in the respective rows with first and secondsizes are alternately arranged adjacent to each other in thecorresponding rows. When the light emitting cell formed at any positionin the first row has the first size, the light emitting cell formed at aposition in the second row corresponding to the position of the lightemitting cell in the first row has the second size, whereby the lightemitting cells in an LED with a confined area can be effectivelyarranged.

Moreover, according to the present invention, in a case where aplurality of LEDs having such configuration and characteristics areconnected to one another to form a light emitting device, respectivelight emitting cells have different turn-on voltages when the respectiveLEDs emit light under AC power, so that times when the respective lightemitting cells start emitting light are different. Accordingly, thelight emitting device can be used for an illumination or light source inwhich a flicker phenomenon is effectively reduced.

Here, if the LEDs are formed on one substrate when the respective LEDsare formed in the light emitting device, it is possible to reduce entirefabrication processes of the light emitting device and to easily performthe operation of adjusting the turn-on voltage of each light emittingcell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing an arrangement of light emitting cells in aconventional LED.

FIG. 2 is a graph showing turn-on voltages of the respective lightemitting cells in the conventional LED.

FIG. 3 is a view showing an arrangement of light emitting cells in anLED according to an embodiment of the present invention.

FIG. 4 is a graph showing turn-on voltages of the respective lightemitting cells in the LED according to the embodiment of the presentinvention.

FIG. 5 is a sectional view illustrating a light emitting deviceaccording to the embodiment of the present invention.

FIGS. 6 to 8 are sectional views illustrating a method of fabricating alight emitting device according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings. Thefollowing embodiments are provided only for illustrative purposes sothat those skilled in the art can fully understand the spirit of thepresent invention. Therefore, the present invention is not limited tothe following embodiments but may be implemented in other forms. In thedrawings, the widths, lengths, thicknesses and the like of elements maybe exaggerated for convenience of illustration. Like reference numeralsindicate like elements throughout the specification and drawings.

FIG. 3 is a view showing an arrangement of light emitting cells in anLED according to an embodiment of the present invention.

Referring to FIG. 3, an LED 100 can be connected to a power supply 200,and emits light under AC power supplied from the power supply 200.

The LED 100 has a plurality of light emitting cells 100-1 to 100-16.

The respective light emitting cells 100-1 to 100-16 are formed bylaminating a multiple semiconductor layers on a substrate andelectrically separating the multiple semiconductor layers into thecells. Each of the light emitting cells 100-1 to 100-16 has a lightemitting structure of a PN junction. Adjacent ones of the respectivelight emitting cells may be electrically connected through metal wires.

The respective light emitting cells 100-1 to 100-16 may be arranged invarious forms. In this embodiment, the light emitting cells 100-1 to100-16 in the LED 100 are arranged in two parallel rows. The first andsecond rows are arranged to have polarities opposite to each other.

Hence, if AC power is applied to the LED 100 from the power supply 200,current flows into the first row in positive voltage intervals such thatthe light emitting cells 11 in the first row emit light, while currentflows into the second row in negative voltage intervals such that thelight emitting cells 11 in the second row emit light.

Therefore, the first and second rows alternately emit light.

The light emitting cells 100-1 to 100-6 in the first row may be formedto have various sizes. In this embodiment, each of the light emittingcells 100-1 to 100-6 has one of two sizes.

In the first row, first, third and fifth light emitting cells 100-1,100-3 and 100-5 have a small size, and second, fourth and sixth lightemitting cells 100-2, 100-4 and 100-6 have a large size.

Since the respective light emitting cells 100-1 to 100-6 are formed tohave a light emitting structure by laminating semiconductor layers madeof the same material on the same substrate, the current densities of thecorresponding light emitting cells are different from each other whenthe same voltage is applied to the light emitting cells if the sizes oflight emitting cells are different from each other.

If the current densities of the light emitting cells are different fromeach other, turn-on voltages at which the corresponding light emittingcells emit light when power is applied thereto are different from eachother.

In general, when the same voltage is applied to a light emitting cell,the current density of the light emitting cell is larger as the size ofthe light emitting cell is smaller. Therefore, since a difference ofturn-on voltages occurs between small- and large-sized light emittingcells as shown in FIG. 4, the turn-on voltage of the small-sized lightemitting cell is lower than that of the large-sized light emitting cell,so that the small-sized light emitting cell emits light at a turn-onvoltage lower than the large-sized light emitting cell.

When AC power is applied, light emitting cells emit light at the turn-onvoltage or more, and stop emitting light at the turn-on voltage or less.

Hence, when AC power is applied, the small-sized light emitting cell isturned on to emit light earlier and maintains a light emitting statelonger than the large-sized light emitting cell. That is, thesmall-sized light emitting cell has a light emitting time longer thanthe large-sized light emitting cell.

By such a principle, if AC power supplied from the power supply 200 isapplied to the LED 100, the first, third and fifth light emitting cells100-1, 100-3 and 100-5 have a lower turn-on voltage in the first rowthan the second, fourth and sixth light emitting cells 100-2, 100-4 and100-6.

Accordingly, although the same AC power is applied, the first, third andfifth light emitting cells 100-1, 100-3 and 100-5 are turned on to emitlight earlier and maintain a light emitting state longer than thesecond, fourth and sixth light emitting cells 100-2, 100-4 and 100-6.

Since small- and large-sized light emitting cells are alternatelyconnected in series in the first row, adjacent ones of the respectivelight emitting cells start emitting light at different times and stopemitting light at different times. According to the embodiment of thepresent invention, a flicker phenomenon is remarkably reduced ascompared with the arrangement of conventional light emitting cells withthe same size.

The light emitting cells 100-11 to 100-16 in the second row in the LED100 may also be formed to have various sizes. In this embodiment, theemitting cells 100-11 to 100-16 are formed to be different in size fromthe light emitting cells 100-1 to 100-6 corresponding thereto,respectively.

That is, in the first row, the first, third and fifth light emittingcells 100-1, 100-3 and 100-5 have a small size, and the second, fourthand sixth light emitting cells 100-2, 100-4 and 100-6 have a large size.On the other hand, in the second row, the eleventh, thirteenth andfifteenth light emitting cells 100-11, 100-13 and 100-15 have a largesize, and the twelfth, fourteenth and sixteenth light emitting cells100-12, 100-14 and 100-16 have a small size.

Light emitting cells in an LED with a confined area can be effectivelyarranged through such an arrangement.

Further, when the first and second rows alternately emit light throughsuch an arrangement, the intensity of light emitted from the lightemitting cells has little difference depending on the position of thelight emitting cell in the LED, so that entire intensity thereof can bemade uniform.

According to the present invention, a flicker phenomenon is remarkablyreduced as compared with the arrangement of the conventional lightemitting cells with the same size.

FIG. 5 is a sectional view illustrating the light emitting deviceaccording to the embodiment of the present invention. For convenience,FIG. 5 is a sectional view illustrating the light emitting diodesarranged in the first row in the light emitting device in FIG. 3.

In addition, the plurality of light emitting cells 100-1 to 100-6 havedifferent sizes from one another but only a cross section thereof isillustrated in this figure, so that the respective light emitting cellsare shown to have the same width. However, although the respective lightemitting cells have the same width, the lengths of the light emittingcells that are not shown in this figure are different from one anotheras shown in FIG. 3.

Referring to FIG. 5, the light emitting device of the present inventioncomprises a heat conductive substrate 110, a buffer layer 120 formed onthe heat conductive substrate 110, a different type semiconductor layerrepeated film 130 having a multi-layered structure formed by alternatelylaminating N-type and P-type semiconductor layers repeatedly, theplurality of light emitting cells 100-1 to 100-6 patterned on thedifferent semiconductor layer repeated film 130, and metal wires 180-1to 180-5 for serially connecting the plurality of light emitting cells100-1 to 100-6 to one another.

The heat conductive substrate 110 is a substrate made of a material withthermal conductivity relatively higher than a sapphire substrate. A SiC,Si, Ge, GeSi, AlN, or metal substrate may be used as the heat conductivesubstrate 110.

The buffer layer 120 is used to reduce lattice mismatch between the heatconductive substrate 110 and semiconductor layers to be formed thereon.Further, in some embodiments of the present invention, the buffer layer120 is used to isolate the light emitting cells 100-1 to 100-6 from theheat conductive substrate 110. The light emitting cells 100-1 to 100-6are electrically spaced apart from one another, so that the buffer layer120 is formed of a semi-insulating material layer.

In this embodiment, the buffer layer 120 may be an AlN orsemi-insulating GaN layer. Since an undoped AlN generally has aninsulating property, the AlN may be an undoped AlN. Meanwhile, anundoped GaN generally has an N-type semiconductor or semi-insulatingproperty depending on a growth method and a substrate material. Hence,when the undoped GaN has a semi-insulating property, the semi-insulatingGaN is an undoped GaN. On the other hand, when the undoped GaN has anN-type semiconductor property, acceptors are doped to cancel the N-typesemiconductor property. The acceptor may be alkaline metal, alkalineearth metal or transition metal, particularly Fe or Cr.

A method of forming a semi-insulating GaN on a sapphire substrate isdisclosed in Applied Physics Letters published on Jul. 15, 2002,entitled “Growth of Fe doped semi-insulating GaN by metalorganicchemical vapor deposition” by Heikman et al. According to Heikman etal., a semi-insulating GaN layer is formed on a sapphire substrate usingan MOCVD technique in which ferrocene (Cp₂Fe) is used as a precursor.

In general, an undoped GaN formed on a sapphire substrate using theMOCVD technique becomes an N-type GaN. This is because remaining oxygenatoms function as donors in a GaN layer. Hence, since the donors arecancelled by being doped with a metallic material that functions asacceptors, such as Fe, a semi-insulating GaN can be formed.

The method of forming a semi-insulating GaN by being doped withacceptors may be equally applied to the embodiment of the presentinvention. For example, the undoped GaN formed on a SiC substrate maybecome an N-type GaN due to impurities including Si or the like. Thus,the semi-insulating GaN buffer layer 120 may be formed by being dopedwith a metallic material such as Fe. At this time, it is unnecessary todope the buffer layer 120 with acceptors throughout the entire thicknessthereof. That is, the buffer layer 120 may be partially doped withacceptors such as Fe by confining a portion of the thickness thereof.

As shown in this figure, the buffer layer 120 may be continuous betweenthe light emitting cells 100-1 to 100-6. However, the buffer layer 120may be discrete.

The different semiconductor layer repeated film 130 is formed byalternately laminating N-type and P-type semiconductor layersrepeatedly.

The number of pairs of alternately laminated N-type and P-typesemiconductor layers ranges from 2 to 500.

In the different semiconductor layer repeated film 130, the N-typesemiconductor layer may be a GaN-based layer doped with N-typeimpurities, e.g., an N-type Al_(x)In_(y)Ga_(1-x-y)N (0≦x, y, x+y≦1).However, the present invention is not limited thereto, but may includevarious semiconductor layers. Further, the P-type semiconductor layermay be a GaN-based layer doped with P-type impurities, e.g., a P-typeAl_(x)In_(y)Ga_(1-x-y)N (0≦x, y, x+y≦1). However, the present inventionis not limited thereto, but may include various semiconductor layers.The N-type and P-type semiconductor layers may be an In_(y)Ga_(1-x)N(0≦x≦1) layer or Al_(x)Ga_(1-x)N (0≦x≦1).

Alternatively, in the different semiconductor layer repeated film 130,the N-type semiconductor layer may be formed by being doped with Si, andthe P-type semiconductor layer may be formed by being doped with Zn orMg.

The N-type and P-type semiconductor layers of the differentsemiconductor layer repeated film 130 are alternately laminated, so thata large potential barrier is formed in a contact surface between theN-type and P-type semiconductor layers. Therefore, current does not flowbetween adjacent semiconductor layers, thereby causing an insulatingeffect.

That is, the light emitting cells 100-1 to 100-6 and the conductivesubstrate 110 are electrically insulated from each other, so thatleakage current that may be generated through the conductive substrate110 can be effectively prevented due to the insulating effect of thedifferent semiconductor layer repeated film 130.

Moreover, the light emitting cells 100-1 to 100-6 can be electricallyisolated from one another due to the insulating effect of the differentsemiconductor layer repeated film 130.

The operation of growing N-type and P-type semiconductor layers in thedifferent semiconductor layer repeated film 130 is more easily performedthan the operation of growing AlN in the buffer layer 120.

Meanwhile, each of the plurality of the light emitting cells 100-1 to100-6 includes a PN-junction nitride semiconductor layer.

In this embodiment, each of the light emitting cells 100-1 to 100-6comprises an N-type semiconductor layer 140, an active layer 150 formedon a predetermined region of the N-type semiconductor layer 140, and aP-type semiconductor layer formed on the active layer 150. At least aportion of the top surface of the N-type semiconductor layer 140 isexposed. Ohmic metal layers 170 and 175 may be formed on the N-type andP-type semiconductor layers 140 and 160, respectively. A semi-metallayer or a highly concentrated N-type semiconductor tunneling layer witha concentration of 1×10¹⁹ to 1×10²²/cm³ may be formed on the N-type orP-type semiconductor layer 140 or 160. Then, a transparent electrodelayer (not shown) may be further formed thereon.

The N-type semiconductor layer 140 may be a GaN-based layer doped withN-type impurities, e.g., an N-type Al_(x)In_(y)Ga_(1-x-y)N (0≦x, y,x+y≦1). However, the present invention is not limited thereto, but mayinclude various semiconductor layers. Further, the P-type semiconductorlayer 160 may be a GaN-based layer doped with P-type impurities, e.g., aP-type Al_(x)In_(y)Ga_(1-x-y)N (0≦x, y, x+y≦1). However, the presentinvention is not limited thereto, but may include various semiconductorlayers.

Each of the N-type and P-type semiconductor layers 140 and 160 may be anIn_(y)Ga_(1-x)N (0≦x≦1) or Al_(x)Ga_(1-x)N (0≦x≦1) layer.

The N-type semiconductor layer 140 may be formed by being doped with Si,and the P-type semiconductor layer 160 may be formed by being doped withZn or Mg.

The active layer 150, which is a region in which electrons and holes arerecombined, includes InGaN. The wavelength of light emitted from a lightemitting cell varies depending on the kind of a material of the activelayer 150. The active layer 150 may be a multi-layered film havingquantum well layers and barrier layers repeatedly formed. The barrierwell layer and quantum well layer may be binary or quaternary compoundsemiconductor layers expressed by a general formulaAl_(x)In_(y)Ga_(1-x-y)N (0≦x, y, x+y≦1).

The light emitting cells are connected in series through the metal wires180-1 to 180-5. In this embodiment, the light emitting cells 100-1 to100-6 are connected in series as many as can be driven by an AC powersupply through the metal wires. That is, the number of light emittingcells to be connected in series is limited by AC driving voltage/currentapplied to the light emitting device and voltage required in driving asingle light emitting cell. For example, about 67 light emitting cellseach of which is driven with 3.3V can be connected in series under an ACvoltage of 220V. Further, about 34 light emitting cells each of which isdriven with 3.3V can be connected in series under an AC voltage of 110V.

As shown in FIG. 5, in the light emitting device having the six lightemitting cells 100-1 to 100-6 connected in series, the N-typesemiconductor layer 140 of the first light emitting cell 100-1 isconnected to the P-type semiconductor layer 160 of the second lightemitting cell 100-2 through the first metal wire 180-1, the N-typesemiconductor layer 140 of the second light emitting cell 100-2 isconnected to the P-type semiconductor layer (not shown) of the thirdlight emitting cell (not shown) through the second metal wire 180-2, theN-type semiconductor layer (not shown) of the fourth light emitting cell(not shown) is connected to the P-type semiconductor layer 160 of thefifth light emitting cell 100-5 through the fourth metal wire 180-4, andthe N-type semiconductor layer 140 of the fifth light emitting cell100-5 is connected to the P-type semiconductor layer 160 of the sixthlight emitting cell 100-6 through the fifth metal wire 180-5.

As disclosed in PCT Patent Publication No. WO 2004/023568(A1), theserially connected light emitting cells form an LED array. Meanwhile,the light emitting device have two LED arrays connected to each other inreverse parallel to be used for illumination under AC power. P-type andN-type pads (not shown) to be electrically connected to an AC powersource may be formed on the P-type semiconductor layer 160 of the firstlight emitting cell 100-1 and the N-type semiconductor layer 140 of thesixth light emitting cell 100-6, respectively.

Hereinafter, a method of fabricating a light emitting device having aplurality of light emitting cells with different sizes will bedescribed.

FIGS. 6 to 8 are sectional views illustrating a method of fabricating alight emitting device according to an embodiment of the presentinvention.

Referring to FIG. 6, a buffer layer 120 is formed on a heat conductivesubstrate 110. The heat conductive substrate 110 may be an AlN or SiCsubstrate. Further, the SiC substrate may be a semi-insulating or N-typesubstrate.

In general, a SiC single crystalline substrate has properties of anN-type semiconductor. This is because nitrogen (N) contained in the SiCsubstrate functions as a donor. Therefore, a semi-insulating SiC singlecrystal can be grown by being doped with acceptors, e.g., vanadium.Meanwhile, a method of growing a semi-insulating SiC single crystalwithout being doped with vanadium is disclosed in U.S. Pat. No.6,814,801. The semi-insulating SiC substrate may be provided using suchmethods.

In order to perform a semiconductor growth process for forming a lightemitting cell, the buffer layer 120 is formed using a method such asMOCVD, MBE, HVPE or the like. The buffer layer 120 may be an AlN orsemi-insulating GaN layer. The semi-insulating GaN layer may be anundoped GaN or a GaN layer doped with acceptors. The acceptor may bealkaline metal, alkaline earth metal or transition metal, particularlyFe or Cr.

At this time, since the buffer layer 120 may be formed to have athickness to the extent that the buffer layer functions as a middlelayer for performing the semiconductor growth process for forming alight emitting cell, it is not required to form the buffer layer up tothe thickness for providing an insulating property.

A different semiconductor layer repeated film 130 is formed on thebuffer layer 120. The different semiconductor layer repeated film 130 isformed to have a multi-layered structure by alternately laminatingN-type and P-type semiconductor layers repeatedly.

The different semiconductor layer repeated film 130 may be continuouslyformed in the same process chamber. Each of the N-type and P-typesemiconductor layers in the different semiconductor layer repeated film130 may be formed using a method, such as MOCVD, MBE or HVPE, and may beformed to have a multi-layered structure.

An N-type semiconductor layer 140, an active layer 150 and a P-typesemiconductor layer 160 are formed on the different semiconductor layerrepeated film 130. These semiconductor layers 140, 150 and 160 may becontinuously formed in the same process chamber. Each of the N-typesemiconductor layer 140, the active layer 150 and the P-typesemiconductor layer 160 may be formed using a method, such as MOCVD, MBEor HVPE, and may be formed to have a multi-layered structure. Asemi-metal layer or a highly concentrated N-type semiconductor tunnelinglayer with a concentration of 1×10¹⁹ to 1×10²²/cm³ may be formed on theN-type or P-type semiconductor layer 140 or 160, and then, a transparentelectrode layer (not shown) may be further formed thereon.

Referring to FIG. 7, the P-type semiconductor layer 160, the activelayer 150 and the N-type semiconductor layer 140 are patterned, therebyforming light emitting cells 100-1 to 100-6 isolated from one another.

At this time, the respective light emitting cells 100-1 to 100-6 havethe same width, but are patterned and isolated to have different lengthsas shown in FIG. 3. Only the same width is expressed in FIG. 7.

The respective layers may be patterned using a photolithography andetching technique. For example, a photoresist pattern is formed on theP-type semiconductor layer 160, and the P-type semiconductor layer 160,the active layer 150 and the N-type semiconductor layer 140 aresequentially etched using the photoresist pattern as an etching mask.Accordingly, the light emitting cells 100-1 to 100-6 isolated from oneanother are formed.

At this time, the different semiconductor layer repeated film 130 may beetched to thereby expose a portion thereof.

In order to prevent electrical insulation between the respective lightemitting cells 100-1 to 100-6 and leakage current through the heatconductive substrate 110, two or more pairs of N-type and P-typesemiconductor layers in the different semiconductor layer repeated film130 are etched from the top thereof, thereby partially exposing thedifferent semiconductor layer repeated film 130.

This is the reason why if the number of laminated N-type and P-typesemiconductor layers is two or more pairs, an insulating effect can besufficiently expected.

As described above, the etching is performed so that the differentsemiconductor layer repeated film 130 is partially exposed, therebyelectrically isolating the light emitting cells 100-1 to 100-6 from oneanother.

Referring to FIG. 8, the P-type semiconductor 160 and the active layer150 of the respective light emitting cells 100-1 to 100-6 isolated fromone another are patterned, thereby partially exposing the top surface ofthe N-type semiconductor layer 140. The patterning process may beperformed using a photolithography and etching process. That is, aphotoresist pattern is formed on the substrate 110 having light emittingcells 100-1 to 100-n isolated from one another, and the P-typesemiconductor layer 160 and the active layer 150 are partially etchedusing the photoresist pattern as an etching mask. As a result, theN-type semiconductor layer 140 is exposed at the etched portions of theP-type semiconductor layer 160 and the active layer 150.

The etching process may be performed through a wet or dry etchingprocess. The dry etching process may be a dry etching process usingplasma.

After the etching process, P-type and N-type ohmic metal layers 170 and175 are formed on the P-type and N-type semiconductor layers 160 and140, respectively.

The ohmic metal layers 170 and 175 may be formed by performing a metaldeposition process after opening regions in which the ohmic metal layers170 and 175 will be formed using a photoresist pattern (not shown).P-type and N-type ohmic metal layers 170 and 175 may be formed throughthe same process or through separate processes. The ohmic metal layers170 and 175 may be formed of at least one material of Pb, Sn, Au, Ge,Cu, Bi, Cd, Zn, Ag, Ni and Ti.

Thereafter, the N-type and P-type ohmic metal layers 175 and 170 ofadjacent ones of the respective light emitting cells are connectedthrough the metal wires 180-1 to 180-5, thereby completing the lightemitting device 100 shown in FIG. 5.

The metal wires 180-1 to 180-5 may be formed through an air bridge orstep-cover process.

The air bridge process is disclosed in PCT Patent Publication No. WO2004/203568(A1) and will be briefly described. First, a firstphotoresist pattern having openings for exposing the ohmic metal layers170 and 175 is formed on the substrate having the light emitting cellsand the ohmic metal layers 170 and 175 formed thereon. Thereafter, ametallic material layer is formed to be thin using an e-beam evaporationtechnique or the like. The metallic material layer is formed on anentire surface of the openings and the first photoresist pattern.Subsequently, a second photoresist pattern is formed to expose regionsbetween adjacent ones of the respective light emitting cells to beconnected and the metallic material layer of the openings. After gold orthe like is formed using a plating technique, the first and secondphotoresist patterns are removed by means of a solution such as solvent.As a result, wires for connecting the adjacent light emitting cells areleft, and the metallic material layer and the photoresist patterns areall removed.

Meanwhile, the step-cover process includes the step of forming aninsulating layer on the substrate having the light emitting cells andthe ohmic metal layers. The insulating layer is patterned using aphotolithography and etching process to thereby form openings forexposing the ohmic metal layers 170 and 175 on the P-type and N-typesemiconductor layers. Subsequently, a metal layer, with which theopenings are filled and which covers a top surface of the insulatinglayer, is formed using an e-beam evaporation technique or the like.Thereafter, the metal layer is patterned using a photolithography andetching process to thereby form wires for connecting the adjacent lightemitting cells. The step-cover process may vary. If the step-coverprocess is applied, wires are supported by an insulating layer, so thatthe reliability of the wires can be improved.

Meanwhile, P-type and N-type pads for being connected to an AC powersource are formed on the light emitting cells 100-1 and 100-6 positionedat both ends of the light emitting device, respectively.

Although the light emitting cells are arrange in a line in thesefigures, it is for the purpose of convenience for illustration. Thelight emitting cells may be arranged in various forms on a plane asdescribed in PCT Patent Publication No. WO 2004/023568(A1).

The present invention has been described with reference to the preferredembodiments and specific modifications. However, it will be understoodby those skilled in the art that a plurality of various otherembodiments different from the aforementioned ones are also included inthe spirit and scope of the present invention.

For example, in the embodiment of the present invention, theconfiguration and characteristics of an LED has been described, in whichlight emitting cells have different sizes and thus different turn-onvoltages from each other when emitting light under AC power, so thattimes when the respective light emitting cells start emitting light aredifferent to thereby reduce a flicker phenomenon. However, when aplurality of LEDs each of which has such configuration andcharacteristics are connected to one another and emit light under ACpower, times when the respective light emitting cells start emittinglight are different to thereby effectively reduce a flicker phenomenonsince the respective light emitting cells have different turn-onvoltages from each other. Accordingly, various light emitting devicescan be fabricated.

Further, it has been described in the embodiment of the presentinvention that when a plurality of light emitting cells are formed in anLED, the plurality of light emitting cells are formed on one substrate.Even when LEDs are formed in a light emitting device, a process can alsobe performed such that the respective LEDs are formed on one substrate.

Furthermore, it has been described in the embodiment of the presentinvention that when a plurality of light emitting cells are formed in anLED, the light emitting cells are different in size from each other.Here, the description that the light emitting cells are different insize may be additionally interpreted as the means that areas occupied bythe corresponding light emitting cells are different. In addition, itcan be also interpreted in the same scope that numerical dimensionsdesigned when light emitting cells are fabricated may vary so thatcurrent densities are different and thus turn-on voltages are differentwhen the same voltage is applied.

1. A light emitting diode (LED) to operate under alternating current (AC) power, comprising: a substrate; a buffer layer arranged on the substrate; and a plurality of light emitting cells arranged on the buffer layer, the light emitting cells comprising different sizes and being spaced apart from one another, the plurality of light emitting cells being connected in series through metal wires.
 2. The LED as claimed in claim 1, wherein the plurality of light emitting cells are arranged so that adjacent light emitting cells that are electrically connected comprise different sizes, and an arrangement of the respective light emitting cells comprising different sizes is repeated.
 3. The LED as claimed in claim 1, wherein the plurality of light emitting cells are arranged in two parallel rows, and polarities of the light emitting cells of the first row and the second row are opposite to each other.
 4. The LED as claimed in claim 3, wherein the light emitting cells arranged in each row comprise a first size and a second size, and the first size cells and the second size cells are alternately arranged adjacent to each other in the corresponding row, and when the light emitting cell arranged at a position in the first row comprises the first size, the light emitting cell arranged at a position in the second row that corresponds to the position in the first row comprises the second size.
 5. The LED as claimed in claim 1, wherein each light emitting cell comprises an N-type semiconductor layer, an active layer, and a P-type semiconductor layer, and the N-type semiconductor layer and the P-type semiconductor layer of adjacent light emitting cells are connected in series through the metal wires.
 6. A light emitting device comprising a plurality of LEDs arranged to operate under alternating current (AC) power, wherein each LED comprises: a substrate; a buffer layer arranged on the substrate; and a plurality of light emitting cells arranged on the buffer layer, comprising different sizes, and being spaced apart from one another, the plurality of light emitting cells being connected in series through metal wires.
 7. The light emitting device as claimed in claim 6, wherein the respective LEDs comprise a common substrate.
 8. An alternating current (AC) powered light emitting diode (LED), comprising: a substrate; a buffer layer arranged on the substrate; and a plurality of light emitting cells, arranged and serially connected by wires, and comprising a plurality of sizes to create variations in the current density characteristics of each light emitting cell and the duration of light emitted from each light emitting cell when the plurality of cells are subject to AC power, and wherein the specific choice of the plurality of sizes of the light emitting cells and their arrangement permit adjacent cells of differing polarity to remain illuminated for varying duration when the AC power is applied, thereby reducing the flicker associated with the application of AC power to adjacent light emitting cells of differing polarity.
 9. The AC powered LED according to claim 1 wherein a ratio of sizes of adjacent light emitting cells is repeated throughout the LED.
 10. The LED according to claim 1 wherein the plurality of light emitting cells is arranged in at least two rows, and a first row and a second row comprise light emitting cells comprising an opposite electrical polarity.
 11. The LED according to claim 10 wherein the light emitting cells arranged in each row have a first size and a second size, and the light emitting cells are alternately arranged adjacent to each other in each row such that when a light emitting cell arranged at a position in the first row comprises the first size, the light emitting cell formed at an adjacent position in the second row comprises the second size. 